Deposits of elemental sulphur have been found in the caprock (consisting mainly of limestone, gypsum, and anhydrite) of a number of salt domes in areas bordering and underlying the Gulf of Mexico; in carbonate and sulphate strata consisting largely of limestone, gypsum, and anhydrite in West Texas and various other regions (notably Iraq, Mexico, and Poland); and in other types of rocks such as diatomites, volcanic tuffs, etc., in other areas.
The elemental sulphur found in the ground was originally deposited in fractures, solution cavities, and/or other types of cavities or void spaces in the caprock, carbonate and sulphate strata, and other types of host rocks. In most cases, a cavity which contains sulphur also has considerable void space. The rocks or formations enveloping or dispersed through the a sulphur deposit also have considerable porosity, commonly ten percent or more. The pores or voids in the rocks can range from pin-hole size to caverns ten feet or more in height. In practically all subsurface sulphur deposits, the rock pores and cavities are occupied or filled by gas, oil, and/or water.
Most of the underground sulphur deposits and their rock environment are overlain by beds of clay/shale or other sediments that contain little or no elemental sulphur. Thickness of such overlying beds usually can be measured in hundreds of feet. Some underground sulphur deposits overlain by appreciable thicknesses of overburden can be mined by conventional underground mining methods. At the present time, however, practically all mining of subsurface sulphur deposits is accomplished by the Frasch or hot water method. Sulphur deposits mined by the Frasch or hot water method include all known deposits of commercial importance in the caprock of salt domes in the U.S.A.; and a number of deposits in carbonate and sulphate strata in West Texas, Mexico, Iraq, and Poland.
The Frasch or hot water method of mining sulphur typically involves the following procedures:
(1) a number of wells, usually located about 100 feet apart, are drilled to the underground sulphur-bearing zones, generally by means of rotary drilling rigs but occasionally with percussion type drilling equipment. Each sulphur production well generally has four concentric pipes or tubular members as follows: (a) a protective tubular casing string extending from the surface to the top of the caprock or sulphur-bearing zone; (b) a tubular pipe string about 6 inches in diameter, with perforations in the lowermost section, extending from the earth's surface to near the bottom of the sulphur-bearing zone; (c) a tubular pipe string about 3 inches in diameter that extends from the earth's surface to a short distance above the bottom of the 6" pipe; and (d) a tubular pipe string about one inch in diameter extending from the earth's surface to a depth somewhat greater than half the depth to the bottom of the 6" pipe string. Other pipe sizes may be used; but this in no way changes the general theory of Frasch mining;
(2) water heated under pressure, to about 325.degree. F., is pumped down the annular spaces between the six-inch and three-inch pipes, and between the three-inch and one-inch pipes during an initial heating period commonly ranging between 24 hours and 96 hours. The superheated water flows out the holes at the bottom of the six-inch pipe into the sulphur-bearing zone and moves upwardly inasmuch as the superheated water has a lower specific gravity than the relatively cool water occupying the voids in the sulphur-bearing zone. The temperature of the sulphur-bearing zone increases, and when it exceeds the melting point of sulphur (about 240.degree. to 245.degree. F.) the sulphur liquifies. Being about twice as heavy as water, the molten or liquid sulphur then flows to the bottom of the well;
(3) at the end of the initial heating period, pumping of superheated water down the three-inch pipe is discontinued. The static pressure of fluid within the sulphur-bearing zone will force the molten sulphur several hundred feet up the three-inch pipe. Then compressed air is then pumped down the one-inch pipe to aerate the molten sulphur and lighten it so that the aerated sulphur will rise to the surface and flow into collecting vessels.
Each production well recovers sulphur from only a small area, usually less than 0.5 acre in extent; and the average producing life of a single well is only 3-4 months. Consequently new wells must be drilled continually as long as a deposit is mined by the Frasch method.
In the Frasch process, provision generally must be made for removing water as well as sulphur from the sulphur deposit and the contiguous porous zones in order to relieve the subsurface fluid pressure that otherwise would become excessive during the course of Frasch mining. The fluid pressure within the sulphur deposit and contiguous porous zones usually increases during Frasch mining because the volume of superheated water that is injected for mining purposes exceeds the volume of sulphur extracted; and the low-permeability strata above and below the sulphur deposit and the contiguous porous zones prevents or restricts the escape of fluids from those zones. Pressure relief is accomplished by drilling "bleed wells", usually located near the periphery of the sulphur deposit, and at structurally low positions, to remove relatively cool water from the base part of the sulphur deposit or underlying porous zone.
The pores and cavities in the sulphur-bearing zone contain fluid which is cooler and has higher specific gravity than the injected superheated water. Consequently, the injected superheated water rises by convection after it exits from the six-inch pipe near the bottom of the well and contacts the cooler water. Although the superheated water cools somewhat as it gives up its heat to the melting sulphur and host rocks and fluids, it is still very hot when it enters porous barren zones overlying and/or contiguous with the sulphur-bearing zone. In fact, temperatures in the range of 220.degree. F. to 290.degree. F. can be found in porous barren zones overlying the sulphur deposit zones from which sulphur is being mined or has been mined by the fusion method; whereas prior to the fusion mining operations the temperatures of porous barren zones usually were less than 100.degree. F.
The thickness of the porous barren zones overlying the sulphur-bearing zones in various deposits mined by a fusion method ranges to several hundred feet. In some cases, the porous barren zones include beds of sand or gravel with great lateral extent. Thus, enormous volumes of hot water, representing vast amounts of wasted energy, are lost to the porous barren zones overlying and/or contiguous with the sulphur-bearing zones mined by the fusion method. In many Frasch mines, 80% or more of the heat energy consumed is lost through escape of hot water to porous barren zones overlying and/or contiguous with the sulphur-bearing zone.
Heretofore, various means of preventing or restricting the escape of hot water from the sulphur-bearing zone have been used or proposed. Such means and related patents are listed below:
(1) injection of sawdust or other comminuted material into the barren zone (U.S. Pat. No. 870,620);
(2) injection of air or other gas to displace water from upper part of caprock (U.S. Pat. Nos. 1,401,593; 1,750,136; and 2,896,932);
(3) introduction of sand into well bore and caprock, from overlying strata (U.S. Pat. No. 1,573,026);
(4) injection of mud and into caprock (U.S. Pat. No. 1,628,873);
(5) replacing the water naturally in the sulphur deposit with brine or other liquid of comparatively heavy specific gravity (U.S. Pat. No. 1,648,210);
(6) emplacement of an impervious barrier (aerated cement) between sulphur-bearing zone and overlying porous barren zone (U.S. Pat. No. 2,784,954);
(7) introducing a silicate solution into a porous zone adjacent to a sulphur-bearing zone, and allowing the solution to set into a relative firm and impermeable mass (U.S. Pat. No. 3,623,700).